German Patent (DE-PS) 3,200,637 C2 (Dekker), published on Aug. 12, 1982, discloses an air jet loom equipped with a weft thread tensioning and detecting device that is arranged downstream of the first weft stop motion device that is integrated into the reed in the area of the weft thread exit of the reed.
The mechanism of the just mentioned German Patent includes a weft stretcher (9) that is effective in a direction cross-wise to the weft insertion direction for stretching the weft thread and keeping the weft thread taut. For this purpose, the weft stretcher which tensions the weft thread establishes an auxiliary airstream that deflects the leading end of an inserted weft thread protruding from the reed perpendicularly to the initial weft insertion direction, whereby the deflected leading weft end extends in a direction cross-wise to the main insertion direction. A first weft stop motion device (8) is positioned near the exit end of the weft insertion channel in the reed. A second weft stop motion device (15) is integrated into a weft guide member superimposed on the weft stretcher (9). The second weft stop motion device (15) provides a signal to the loom control if the inserted weft thread exceeds its rated length by a predetermined value as may be the case when a weft thread breaks. However, in order to prevent that an excess of the rated length of the weft thread causes the second weft stop motion device to generate a fault signal even though no weft break is present, and the exceeding of the rated length is merely due to an excess stretching response of the weft thread during the weft thread insertion operation, the spacing between the second weft stop motion device (15) and the stretcher (9) can be selected large enough so that such a weft thread that is temporarily stretched cannot reach the second weft stop motion device. However, the device disclosed in DE-PS 3,200,631 C2 is not suitable due to its structure, to detect a weft thread break of weft threads which are not to be stretched as is the case, for example for a portion of the weft threads used in weaving certain fabrics, wherein some weft threads are stretched and some are not to be stretched.
U.S. Pat. No. 5,226,458 (Bamelis), issued on Jul. 13, 1993, discloses a device for stretching a weft thread in weaving machines. The device is positioned at the exit end of the weft insertion channel in the reed (2) and connected to a sley (1) just as the reed itself. The known stretching device has a weft thread entrance leading into a weft guide channel extension in the direction of the weft thread insertion motion, whereby the weft guide channel is positioned in a first plane of the device.
The weft stretching device comprises two air flow ports positioned opposite each other across the weft guide channel. Due to this arrangement the stretching airstream extends at an angle cross-wise to the weft insertion direction in the guide channel and flows into a second flow channel arranged in a second plane above the first mentioned plane, thereby forming a second weft guide channel. Corresponding weft stop motion devices positioned for cooperation with the two weft guide channels are not disclosed in the above mentioned U.S. patent. The just described conventional arrangement of the weft stretching or tensioning device has the disadvantage that it cannot accommodate the stretching characteristics of different types of weft threads since the device is not variable in its length in the direction of weft insertion. Thus, there is room for improvement.